Image pick-up apparatus and image pick-up system

ABSTRACT

An image pick-up apparatus and an image pick-up system constructed to prevent occurrence of random noise in a photographed image due to a random noise component produced in a reference supply circuit. An image pick-up apparatus has an area sensor driven by matrix driving, and a reference supply circuit for supplying a reference voltage for driving of the area sensor, and the reference voltage is supplied through a low-pass filter (LPF) coupled to the reference supply circuit. Further, a cutoff frequency of the low-pass filter is preferably determined so that an effective value of noise of the reference voltage having passed through the low-pass filter becomes not more than one-tenth of an effective value of random noise produced in pixels of the area sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image pick-up apparatus such asradiographic apparatus and the like and, more particularly, to imagepick-up apparatus and an image pick-up system provided with a referencesupply circuit for supplying a reference voltage for driving of an areasensor.

2. Related Background Art

A conventional example of image pick-up apparatus will be describedbelow with the drawings. FIG. 7 is an equivalent circuit diagram of theconventional example and FIGS. 8A and 8B are diagrams showing its drivetiming and output. As shown in FIG. 7, the conventional apparatus isprovided with an area sensor in which pixels p1.1-3.3, each consistingof a photodiode as a detecting element and a TFT as a switching devicefor the photodiode, are arranged in a two-dimensional array and which isdriven by matrix driving; a gate drive device 2 for driving the TFTs ofthe area sensor; a read device 1 to which signals outputted by thedriven TFTs are transferred; and a reference supply circuit 4 forsupplying reference voltages for driving or the like of the area sensorand other devices.

A gate electrode of the TFT in each pixel is coupled to a common gateline Vg1-Vg3, and the common gate lines are coupled to the gate drivedevice 2 comprised of an unrepresented shift register and the like. Asource electrode of each TFT is coupled to a common data line sig1-sig3to be coupled to the read device 1 comprised of amplifiers, sample holdcircuits, an analog multiplexer, and so on.

In the present conventional example the reference supply circuit 4supplies the following reference voltages.

-   -   Sensor bias voltage (Vs)    -   Sample hold reference voltage (VREF1)    -   Amp reset voltage (VREF2)    -   TFT on voltage (Von)    -   TFT off voltage (Voff)

Among these, the sensor bias voltage (Vs) is supplied directly from thereference supply circuit 4 to the area sensor. The sample hold referencevoltage (VREF1) is supplied to the sample hold circuits of the readdevice 1, and the amp reset voltage (VREF2) to the amplifiers of theread device 1. The TFT on voltage (Von) and off voltage (Voff) aresupplied through the gate drive device 2 to the area sensor.

The operation will be described below with the timing chart of FIG. 8A.A reading operation for one line will be described. In FIG. 8A, RESrepresents a reset signal to the amplifiers of the read device 1, Vg1-3timing signals of a gate pulse to the respective common gate lines, SMPLa sampling timing signal to the sample hold circuits of the read device1, and CLK an analog synchronization signal from the analog multiplexerof the read device 1.

The amplifiers are first reset, the TFTs are then turned on (Vg1),signal charges in the photodiodes of p1.1-p3.1 are transferred therebyto the amplifiers of the read device, the sample hold circuits (SMPL)sample and hold the charges, and the analog multiplexer provides ananalog output thereof in synchronism with CLK. FIG. 8B is a diagramshowing the output.

As described above, the conventional image pick-up apparatus is providedwith the reference supply circuit to supply the reference voltages tothe sensor, the gate drive device, the read device, and so on. FIG. 9 isa circuit diagram showing an example of the reference supply circuit inthe conventional image pick-up apparatus. A device for regulating eachreference voltage of the sensor bias voltage Vs, the TFT off voltageVoff, the TFT on voltage Von, and the amp reset voltage VREF2 iscomprised of a 3-terminal regulator (e.g., TL317 available from TexasInstruments).

However, the supply circuit such as series regulator IC or the like hasa random noise component due to flicker noise and thermal noise ofsemiconductors. For example, when the reference supply circuit isconstructed of AD780 being the series regulator IC available from AnalogDevices, the random noise component is approximately 100 nV/√{squareroot over (Hz)}.

In the conventional image pick-up apparatus, this random noise componentof the reference supply circuit sometimes caused line noise in an image.With variation in the amp reset voltage VREF2, the sample hold referencevoltage VREF1, the TFT off voltage Voff, and the sensor bias voltage Vsin FIG. 7, there occurred the line noise as shown in the “actual analogsignal” in FIG. 8B in certain cases. The line noise is given by thedifference between the “ideal analog output” and the “actual analogsignal” in FIG. 8B. Such line noise appeared in a stripe pattern in animage reproduced from the signal read by the area sensor, and degradedthe quality of the image in some cases.

Particularly, high frequency components out of noise components of thereference voltages become beatlike line noise through sampling with theSMPL signal and are considered to have great effect on degradation ofimage quality. The occurrence of these line noise sometimes posed aproblem in particular in the case where it was necessary to acquire theimage data with a high degree of accuracy, e.g., in the case of X-rayimage pick-up apparatus and the like.

The present invention has been accomplished in view of the above pointand an object of the present invention is to provide image pick-upapparatus and an image pick-up system constructed to prevent theoccurrence of the line noise due to the random noise componentoriginating in the reference supply circuit for supplying the referencevoltages.

SUMMARY OF THE INVENTION

In order to solve the above problem, the present invention provides animage pick-up apparatus comprising an area sensor driven by matrixdriving, and a reference supply circuit for supplying a referencevoltage for driving of the area sensor, wherein the reference voltage issupplied through a low-pass filter (LPF) coupled to the reference supplycircuit.

A cutoff frequency of the low-pass filter is determined so that aneffective value of noise of the reference voltage having passed throughthe low-pass filter becomes not more than one tenth of an effectivevalue of random noise produced in pixels of the area sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the reference supply circuit inEmbodiment 1 of the present invention;

FIG. 2 is a circuit diagram of the reference supply circuit inEmbodiment 2 of the present invention;

FIG. 3 is a schematic illustration to show a configuration of anapplication example of Embodiment 1 or 2 to an image pick-up system;

FIG. 4 is a sectional view in a one-pixel area of an X-ray image pick-upapparatus used in Embodiment 3;

FIG. 5 is a circuit diagram to show a specific circuit example forbiasing photosensors;

FIG. 6 is a sectional view in a one-pixel area of the X-ray imagepick-up apparatus used in Embodiment 3;

FIG. 7 is an equivalent circuit diagram of the ordinary image pick-upapparatus;

FIGS. 8A and 8B are a timing chart showing the driving of the ordinaryimage pick-up apparatus and a graph showing its output;

FIG. 9 is an equivalent circuit diagram of the reference supply circuitin the conventional example;

FIG. 10 is a graph to show the MTF characteristics of human visualsense; and

FIG. 11 is a graph to show the limitation characteristics of line noiseobtained from the visual sense characteristics.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to the accompanying drawings.

Embodiment 1

FIG. 1 is a schematic equivalent circuit diagram of the reference supplycircuit in the present embodiment. The drive timing and others in thepresent embodiment can be similar to those in the conventional example.

In the present embodiment, low-pass filter (LPF) circuits are added tooutputs of regulator IC. The DC output 101 from the DC/DC converter (forexample, DC/DC converter of the FIG. 9), the DC power, etc. is outputtedby IC102(a)-102 (d). The random noise of the regulator IC appears in theform of line noise in an image and can degrade the image quality.Particularly, high-frequency noise components provide great negativeeffect on the image quality. This is because the effective value ofthermal noise of the regulators is determined by the product of noisedensity N (V/√{square root over (Hz)}) and the bandwidth B (√{squareroot over (Hz)}).

Namely, an effective means for reducing the effective value of noise ofthe regulators to decrease the line noise is to couple the low-passfilter (LPF) circuits to the outputs of the regulators as in the presentembodiment. In this case, the cutoff frequency of LPF is desirablydetermined according to a required response time for each referencevoltage.

The following presents examples of the “required response time for eachreference voltage” stated herein.

-   -   A time between supply of power from a DC/DC converter or the        like to the reference supply circuit and stabilization of the        voltage level of each reference voltage at a certain level.    -   Or a time between supply of power from the DC/DC converter or        the like and establishment of a state of being ready for        acquisition of an image. An interval of acquisition of image,        i.e., a frame rate (particularly, in the case of the voltage        such as the sensor bias voltage Vs or the like being changed by        an unrepresented control circuit or the like), or the cutoff        frequency of LPF is desirably a frequency lower than a sampling        frequency determined by the period of the SMPL signal.        Specifically, the cutoff frequency of LPF is set to be 3 to 10        times the frame rate, which is effective to the line noise. In        the case of photography of still images (e.g., approx. 2        frames/sec), it is preferable that fc (cutoff frequency)=6 to 20        Hz. When the number of frames is large as in the case of moving        pictures or the like, for example, in the case of 30 frames/sec,        it is preferable that fc=90 to 300 Hz.

It is verified from the result of experiment that it is hard for a manto visually perceive the line noise of the image pick-up apparatus whenit is not more than one tenth of the random noise in pixels.Accordingly, the cutoff frequency of LPF is desirably determined so thatthe effective value of noise of each reference voltage becomes not morethan one tenth of the effective value of the random noise in pixels.

In the present embodiment the LPFs are provided for the respectivereference voltages of the sensor bias Vs, TFT off Voff, TFT on Von, andamp reset VREF2, but the LPFs may be provided for part of thesereference voltages or for other reference voltages. The cutofffrequencies of the respective LPFs may be equal to or different fromeach other. The following will describe the effect achieved by theprovision of the LPFs.

It is well known that the human visual sense characteristics are bandpass characteristics for spatial frequencies. Namely, as shown in FIG.10, the human visual sense has such characteristics that a peak ofvisual sense exists at the spatial frequency of about one line pair/mmat the distance of distinct vision of 25 cm and the visual sense islower in the lower frequency and higher frequency areas than the peak.At the spatial frequency of dc, i.e., 0 line pair/mm, the visual sensecharacteristics become approximately zero, so that almost nothing isseen.

Since the line noise in the image should not be seen by an observer, itis desirable that the line noise should possess characteristics matchingthe human visual sense characteristics. In consideration of thelimitation of the line noise, the hardest point is the spatial frequencycorresponding to the peak of the human visual sense characteristics.

Since the visual sense characteristics are lowered at the lower spatialfrequencies than the peak, the limitation can be relaxed there. Namely,since the human visual sense characteristics can be employed as alimitation to the perception of line noise, the limitationcharacteristics of line noise become reverse to the visual sensecharacteristics, as shown in FIG. 11, and are thus such low-passcharacteristics that the limit is infinity at the spatial frequency ofdc, i.e., 0 line pair/mm and is minimum at the peak frequency of visualsense.

Electrical temporal frequencies are sampled and held line by line in thesensor and thereafter reconstructed as an image; that is, theycorrespond in one-to-one correspondence to spatial frequencies of theimage. Therefore, insertion of an electrical low-pass filter isequivalent to insertion of a spatial low-pass filter in the image,whereby the aforementioned effect can be achieved.

Embodiment 2

FIG. 2 is a schematic circuit diagram of the reference supply circuit inthe present embodiment. The equivalent circuit diagram, the drivetiming, etc. in the present embodiment are similar to those in theconventional example.

A point to be noted in the present embodiment is that amplifiers arefurther coupled to the outputs of the LPFs in Embodiment 1. This methodis effective in the case where the reference voltages need to besupplied in low impedance.

It is, however, necessary to pay attention to selection of theamplifiers coupled to the outputs of the LPFs when the configuration ofthe present embodiment is employed. In order to reduce the line noise,it is desirable to select the amplifiers with the noise density of notmore than 3.3 nV/√{square root over (Hz)}. (It is particularly desirableto select them in the frequency region of 100 Hz to 100 kHz.) Describingthis in further detail, the random noise of the area sensor used in theX-ray image pick-up apparatus is normally approximately several hundredμVrms. Since the line noise is preferably controlled to not more thanone tenth of the random noise as described previously, the line noiseneeds to be not more than several ten μVrms. Accordingly, the sum ofcontribution of the LPF circuits to the line noise and contribution ofthe amplifiers to the line noise is preferably not more than several tenμVrms. For example, supposing that the contribution of the amplifiers tothe line noise is set to not more than 1 μVrms and the frequency regionof the amplifiers is 100 Hz to 100 kHz as described above, the noisedensity of the amplifiers is NnV/√{square root over (Hz)}×√{square rootover (100 kHz)}<1 μVrms, and by deforming this relation, we obtain N<3.3nV/√{square root over (Hz)}.

Embodiment 3

FIG. 3 shows an application example of the above embodiments to theimage pick-up system. The present embodiment is an X-ray image pick-upsystem for taking an X-ray image, and the above embodiments are appliedto X-ray image pick-up apparatus 6040. An X-ray tube 6050 as an X-raygenerator generates X-rays 6060, the X-rays 6060 travel through anobserving portion 6062 such as the chest part or the like of a patientor subject 6061, and they are then incident to the X-ray image pick-upapparatus 6040. The incident X-rays carry information about the interiorof the subject 6061. The X-ray image pick-up apparatus 6040 acquireselectrical information in correspondence to the incidence of X-rays.This information is converted into digital data, the digital data issubjected to image processing in an image processor 6070 as an imageprocessing means, and the resultant image can be observed on a display6080 as a display means placed in a control room.

This information can also be transferred through a transmission meanssuch as a phone line, a radio link 6090, or the like to a remote place,whereby it is feasible to display the image on a display 6081 or outputthe image on a film or the like in a doctor room at a different place,thereby allowing a doctor at a remote place to make diagnosis. Theinformation obtained can also be recorded or stored in recording mediausing various recording materials such as an optical disk, amagnetooptical disk, a magnetic disk, etc. or in a recording medium 6110such as a film, paper, or the like by means of a recording means 6100such as a film processor or the like.

The image pick-up apparatus in the image pick-up system used innondestructive inspection, e.g., for medical diagnosis or for internalinspection, is required to read the image with a high degree ofaccuracy. Embodiments 1 and 2 are suitably applicable to suchapplication, because the effect of the line noise is reduced.

FIG. 4 is a sectional view in one pixel of the X-ray image pick-upapparatus of the present embodiment. FIG. 4 shows an example in whichthe incident X-rays are converted into light and the light is detected,but it is also possible to use the image pick-up apparatus for directlydetecting the X-rays as shown in FIG. 6.

One pixel is comprised of a photosensor 401 such as a photodiode or thelike, and a switching device 402 such as a TFT or the like, and suchpixels are formed in a matrix pattern on a glass substrate 403. Aphosphor layer 405 for converting incident X-rays into light is providedthrough a protective layer 404 on the pixel. The photodiode and TFT arepreferably made of amorphous silicon or polysilicon, and the phosphor ispreferably cesium iodide or a gadolinium base material. A preferableconfiguration is one using an MIS (Metal-Insulator-Semiconductor) deviceas the photosensor and a TFT as the switching device, because they canbe produced in the same simple process. In the case of thisconfiguration, it is preferable to prepare at least two types of biasesapplied to the photosensor, one as a bias for reading and the other as abias for refreshing (to sweep out carriers accumulated in the device).Specifically, as shown in FIG. 5, a preferred circuit configurationconsists of a DC/DC output supply 501, a regulator 502, a first LPFcircuit 503, a read bias Vs terminal 504, a refresh bias Vref terminal505, a multiplexer 506, an amplifier 507, and a second LPF circuit 50-8.As illustrated, the LPF circuits are provided for respective Vs andVref, the voltages are switched by the multiplexer, and thereafter theyare again amplified by the amplifier 507. The cutoff frequencies of thefirst LPF circuit and the second LPF circuit disposed before and afterthe multiplexer 506 are preferably approximately equal to each other.The reason for it is that if either one of the frequencies is setrelatively lower the lower cutoff frequency will become dominant.

In the case where the biases applied to the photosensor are switched tobe used as in the present embodiment, the time of switching from Vs toVref in one frame, the response time from Vs to Vref, needs to be set soas not to affect the frame rate. For example, when the frame rate is setto the frame rate in the case of photography of still images (e.g., inthe case of approximately 2 frames/sec), it is preferable to set fc=20to 2000 Hz.

FIG. 6 is a sectional view in one pixel of the X-ray image pick-upapparatus in another embodiment. A pixel is constructed in aconfiguration wherein on a glass substrate 601 a drive substratecomprised of a switching device 602 such as a TFT or the like and acapacitor 603 for storing a signal charge is electrically coupledthrough a connection bump 604 to a radiation converting section (sensordevice section) composed of a lower electrode 605, a conversion layer606 for directly converting radiation such as X-rays or the like to anelectric charge, and an upper electrode 607. In this configuration, thelower electrode 605 for each pixel is separated from others, and theseparate electrode zones are arranged in a two-dimensional array. Inthis example the pixel is composed of the semiconductor material fordirectly converting X-rays into an electric charge, the storagecapacitor and TFT coupled thereto, and so on. Just as in the case of theexample of FIG. 4, the TFT is preferably made of amorphous silicon orpolysilicon. Further, the X-ray-to-electron converting layer can be madeof a material selected from gallium arsenide, gallium phosphide, leadiodide, mercury iodide, amorphous selenium, CdZn, CdZnTe, and so on.

When the apparatus is constructed in the configuration wherein the areasensor part is formed on the insulating substrate and the read circuit,the LPF circuits, etc. are made of ordinary crystalline silicon as inthe present embodiment, it is feasible to decrease the time necessaryfor processing of detected signals. Since the sensor part (drive circuitsubstrate) is the insulating substrate, the above configuration ispreferable, because the noise can be reduced, particularly, inapplication as radiation image pick-up apparatus.

1-18. (canceled)
 19. An image pick-up apparatus comprising; an areasensor in which detecting elements are two-dimensionally arranged on asubstrate; a reference supply circuit, for supplying a reference voltageto the detecting elements, which comprises a regulator for regulatingthe reference voltage; and a low-pass filter arranged between saidregulator and said detecting elements, wherein the reference voltage issupplied through said low-pass filter from said reference supplycircuit.
 20. The image pick-up apparatus according to claim 19, whereina cutoff frequency of said low-pass filter is determined according torequired response time for the reference voltage.
 21. The image pick-upapparatus according to claim 19, wherein a cutoff frequency of saidlow-pass filter is lower than a sampling frequency of a sample holdcircuit coupled to said area sensor.
 22. The image pick-up apparatusaccording to claim 19, wherein an amplifier is placed on the output sideof said low-pass filter.
 23. The image pick-up apparatus according toclaim 22, wherein a noise density of said amplifier is less than 3.3nV/√{square root over (Hz)}.
 24. The image pick-up apparatus accordingto claim 19, wherein said area sensor comprises a two-dimensional arrayof pixels, each consisting of a detecting device and a switching device,wherein said detecting device is a photoelectric conversion device andsaid switching device is a TFT, and wherein said photoelectricconversion element is coupled to a drain electrode of said TFT, a gateelectrode of said TFT to common a gate line, a source electrode of saidTFT to a common data line, said common data line to a read device, andsaid common gate line to a gate drive device, thereby implementing amatrix operation.
 25. The image pick-up apparatus according to claim 24,wherein said read device has an amplifier coupled to said common dataline and is capable of reset operation, and wherein an amplifier resetvoltage as a reference voltage of said amplifier is supplied theretofrom said reference supply circuit.
 26. The image pick-up apparatusaccording to claim 24, wherein a TFT on voltage and a TFT off voltage asa reference voltage of said TFT are supplied to said gate drive devicefrom said reference supply circuit.
 27. The image pick-up apparatusaccording to claim 24, wherein a bias voltage for applying a biasvoltage to said photoelectric conversion element, as said referencevoltage, is supplied to said area sensor.
 28. The image pick-upapparatus according to claim 24, wherein said photoelectric conversionelement and TFT are made of amorphous silicon or polysilicon.
 29. Theimage pick-up apparatus according to claim 24, wherein saidphotoelectric conversion element is a pin photodiode or MIS sensor. 30.The image pick-up apparatus according to claim 24, wherein said areasensor is an X-ray area sensor further comprising a phosphor.
 31. Theimage pick-up apparatus according to claim 19, wherein said substrate isan insulating substrate and said reference supply circuit and LPF areformed on a crystalline substrate.
 32. An image pick-up systemcomprising the image pick-up apparatus as set forth in claim 19, imageprocessing means for processing a signal from said image pick-upapparatus into an image, recording means for recording a signal fromsaid image processing means, display means for displaying a signal fromsaid image processing means, and transmitting means for transmitting asignal from said image processing means.
 33. The image pick-up systemaccording to claim 32, further comprising a wavelength conversionelement for converting radiation into wavelengths in a sensitive regionof said sensor elements.
 34. An image pick-up apparatus comprising: anarea sensor in which detecting elements are two-dimensionally arrangedon a substrate; a read device for reading a signal from said detectingelements, which comprises an amplifier for amplifying the signal; areference supply circuit, for supplying a reference voltage to theamplifier, which comprises a regulator for regulating the referencevoltage; and a low-pass filter arranged between said regulator and saidamplifier, wherein the reference voltage is supplied through saidlow-pass filter from said reference supply circuit.
 35. An image pick-upapparatus comprising: an area sensor in which detecting elements aretwo-dimensionally arranged on a substrate; a read device for reading asignal from said detecting elements, which comprises an amplifier foramplifying the signal; a reference supply circuit, for supplying a firstreference voltage to said detecting elements and for supplying a secondreference voltage to said amplifier, which comprises a first regulatorfor regulating the first reference voltage and a second regulator forregulating the reference voltage; a first low-pass filter arrangedbetween said first regulator and said detecting elements, wherein thefirst reference voltage is supplied through said low-pass filter fromsaid reference supply circuit; and a second low-pass filter arrangedbetween said second regulator and said amplifier, wherein said secondreference voltage is supplied through said second low-pass filter fromsaid reference supply circuit.